1. Field of the Invention
The invention relates to an image forming apparatus.
2. Description of the Related Art
Various modes can be set, hitherto, in image forming apparatus operating on electrophotographic system such as color copying machines and color printers. Specific modes include color, monochromatic, color-monochromatic hybrid, one-side copy, two-side copy, face-down (FD) discharge for discharging the print side down, and face-up (FU) discharge for discharging the print side up.
Such image forming apparatus is shown in FIG. 1. As shown in FIG. 1, the image forming apparatus includes a rotatable photosensitive drum 1 as an image bearing member. A charging roller 2 is disposed above the photosensitive drum 1. The charging roller 2 is a roller for uniformly charging the surface of the photosensitive drum 1.
A laser unit 3 is writing means, which forms an electrostatic latent image by selectively exposing the surface of the photosensitive drum 1 depending on image signal. At the left side of the photosensitive drum 1, a development unit 4 is provided as developing means. The development unit 4 is a device for rendering the electrostatic latent image into a sensible image by using a toner.
The development unit 4 consists of four units 5Y, 5M, 5C, SK containing toners of four colors, yellow, magenta, cyan, and black, respectively. Three development units 5Y, 5M, 5C out of four units 5Y, 5M, 5C, 5K are provided in a rotary development device 6.
The color development units 5Y, 5M, 5C provided in the rotary development device 6 sequentially face the photosensitive drum 1, and develop. Then, the black development unit 5K always sitting oppositely to the photosensitive drum 1 operates and develops.
An intermediate transfer belt 7 is provided beneath the photosensitive drum 1. On the intermediate transfer belt 7, sensible toner images developed by the color development units are sequentially transferred by a primary transfer unit T1, and multiple transferred toner images are formed on the surface.
A sheet P is supplied onto a registration roller 8 from a feed unit The sheet P waiting on the registration roller 8 is sent into a secondary transfer unit T2 in synchronism with the toner image on, the intermediate transfer belt 7.
The secondary transfer roller 9 is movable to contact with or depart from the intermediate transfer belt 7. The secondary transfer roller 9 is apart during multiple transfer process by the intermediate transfer belt 7, and is in contact during secondary transfer process. By this contacting motion, the toner image is transferred on the sheet P by the secondary transfer unit T2.
The toner image carried on the sheet P is heated and pressed by a fixing roller 10, and is fixed on the sheet P. Then, the sheet P on which the toner image is formed is discharged on a discharge tray 26 by a discharge roller 17.
The discharge unit includes a flapper 16, a discharge flapper 21, an inversion flapper 22, and a motor (not shown) changeable in rotating direction. By these flappers 16, 21, 22 and motor, the conveying path and conveying direction can be changed over, and it is applicable to various modes, including one-side and two-side print, FU discharge, and FD discharge.
In the image forming apparatus having such configuration, hitherto, cooling means such as a fan is used to lower the temperature in the product main body. To cool the area around the fixing roller 10 which releases mass of heat structurally or the photosensitive drum 1 which is delicate to high heat, an exhaust fan 13 is installed to force out the heated air.
Recently, the quality is much enhanced in a printer and a copying machine, and it is demanded to form images on a thick coated paper. In the conventional structure, when forming an image on thick coated paper, heat capacity of thick coated paper is very large, the sheet is heated by the fixing roller, and conveyed into the discharge unit in very hot state.
As shown in FIG. 6, the toner is not fixed on the sheet, and the soft toner contacts with the conveying roller or rib 18 of paper conveying guide disposed in the conveying path, and roller marks and rib marks are formed, and uneven gloss occurs. Besides, since a hot sheet is discharged, the soft toner plays the role of an adhesive, and the sheets are stuck together on the discharge tray in a worst case.
Accordingly, it has been proposed to cool the sheet and harden the toner by blowing air drawn from outside of the product on the sheet before the sheet contacts with the conveying roller or conveying guide, or before it is discharged into the discharge tray (see Japanese Patent Application Laid-Open (JP-A) No. 11-352840).
However, in the technology disclosed in JP-A No. 11-352840, by installing a sheet cooling fan in the discharge unit, undesired air flow may be formed in the image forming apparatus. That is, the air for cooling the sheet is desired to be low in temperature. Accordingly, instead of inside air of product high in temperature, air outside of product is taken in and blown to the sheet.
Thus, the sheet cooling fan draws air from outside of the apparatus main body, resulting in positive pressure air in the discharge unit. As a result, air around the fixing unit adjacent the discharge unit enters into the image forming unit.
As a result, heat of the fixing roller controlled around 200 C. is transmitted to the image forming unit, and the temperature of image forming unit is elevated. In particular, if the transfer cleaner often installed near the fixing unit is raised in temperature, the temperature of transfer cleaner may exceed the softening point of toner (50 to 60 C.), and toner clogging may occur. By toner clogging, waste toner may not be collected in the cleaner. Finally, the toner overflows and stains the image.
This problem becomes more serious when a plurality of fans is installed for cooling the sheets, or a large volume of air is used for cooling the sheets. Or if the exhaust of air supplied form the discharge cooling fan is directed toward the fixing roller, the fixing roller is cooled. As a result, the duty of fixing heater becomes high, and the power consumption is increased.